Filters are not performance equipment, they are protective equipment. You won’t find a filter that flows air better (and makes more power) than a clean stock filter. Unfortunately, the stock filter elements don’t seal very well to the air runner and don’t filter dirt from the air as well as aftermarket filters.
The best location for the filter is in the air runner, and best filter of this type is the Pipercross MPX038. The Pipercross filters are the same shape as the stock filters, but thicker. They even use the stock plastic frame to give rigidity. Thicker means they seal to the air runners better and hold more dirt. They also filter better since they use a coarse layer (similar to the stock unit in cell size) bonded to a smaller cell foam layer so they capture smaller dirt particles. They come supplied with filter oil. Most important, they don't mess-up your air box resonance.
Filters located in the air box take-up space which changes the resonance of the intake system at various engine speeds. Air box resonance helps to fill the cylinders with air, and it’s important to note that factory engine development work to smooth-out the power and torque curves is done using the stock filters. In particular, air box filters degrade throttle response, the ability to smoothly accept changes in throttle, by reducing the volume of “free” air located in the space between the filter element and the throttle plate. The volume of “free” air should be at least 1.5 liters for the best throttle response for liter displacement bikes.
Ask yourself, why do bikes like the 748R come with larger 14 liter air boxes instead of the stock 8 liters? This before-after dyno comparison of a 996 with a large volume EVR airbox shows why. Notice how it smooths-out the dip in mid-range torque. Air box resonance effects.
Another type of air box filter is installed over the throttle body velocity stack. Some owners choose these to protect the engine from any dirt ingestion in the event of a crash that dislodges the fuel tank, or from a poorly sealed air box.
Peak power is essentially the same for the different filter types - when clean. Dirt-holding capacity depends on a number of factors, so any filter needs to be cleaned regularly, some more often than others. Long Answer
Regarding aftermarket air filters, the short answer is keep the stock filters and keep them CLEAN. The aftermarket filters either don't filter as well and/or they reduce performance.
Modern high performance motorcycle engines are the result of countless hours of development work on a manufacturer's dyno. They are designed as a unit with air flow and exhaust flow optimized together for each engine configuration. Purposely (say to save costs) using an unecessarily restrictive air filter in their design will decrease performance relative to their competitors and their marketing advantage.
Ask yourself this question: Where are the dyno charts from the manufacturers of the aftermarket air filters?
If they really improve performance over stock filters across the rpm range then it's really a marketing advantage to release their design development dyno charts. Without evidence to the contrary, it's safe to conclude that it is not to their advantage to release any dyno charts or comparisons with other vendors. The aftermarket air filter market for motorcycles seems to be built on hype by the manufacturers and by the profits to their sellers. They compete on heresay and testimonies from “happy” customers or recomendations from their own vendors and sponsored racers— not on proof of superior performance.
First, there's the issue of modifying the airbox design.
One reason for using an air box and intake runners is to direct cool air to the engine. This design also results in an small pressurization of intake air that increases with speed. It also reduces the volume of the intake flow noise.
But there's also a performance benefit because the airbox is a Helmholtz resonator. That is, a resonance effect occurs when you connect a suitable enclosed volume to an engine's intake stacks causing the air inside to resonate at a frequency that results in pressure peaks coincident with the cylinders intake strokes. This can increase power by 10–15% within a particular rev range. Airboxes need to be well sealed and stiff in construction to maintain these resonance characteristics.
However, when you fill up a large portion of the airbox volume with an aftermarket foam filter you change the resonance characteristics of the airbox.
Regarding the large foam in-airbox filters, the people at Sigma Performance (www.sigmaperformance.com
) distributers of arguably the best aftermarket chips for superbikes recommended using the stock filters. They state that all FIM chips are made and tuned with the stock filters, and they highly recommend staying with the stock units.
This is because they have found that a new CLEAN stock air filter gives 2–5% better performance than any of the aftermarket filters and they have only seen reductions in performance when using over-the-bellmouth filters. As a type, they change the air box resonance and require about a 3% leaner mixture to get back some of the power. In particular, these filters have flow characteristics that aren't well matched to the fixed metering of the stock fuel injection system.
On the other hand, the main advantage of the of a over-the-bellmouth filter location in the airbox is that it allows only clean air into the motor. The fuel tank-to-airbox seal is notoriously poor on superbikes, and will often allow dirt into the airbox bypassing the stock filters.
So, if you see bits of sand and grit and dead bug parts in the bottom of the airbox one solution is to replace the stock tank-to-airbox seal with thicker stick-on foam tape, or use the updated seal from the 1999 and-up bikes.
Access to the stock filters requires some effort so if you don't clean them regularly an aftermarket filter may be a better choice for you.
Further, over-the-bellmouth designs, however, do reduce the chance of engine damage in case of a crash. IF you crash, AND the gas tank becomes dislodged from the air box breaking the seal, AND debris gets into the air box these designs can prevent engine ingestion and further damage caused by the crash. Your insurance company will thank you.
Finally, seat-of-the-pants tests can be misleading. If an aftermarket filter results in the loss of low and/or mid rpm throttle response, with no gain in peak power, the bike seem quicker (peakier) and that's why you might think they're a good deal. However, when you test two otherwise equal bikes side by side, you'll find that the one with the stock air filter setup will always pull an impressive gap on the aftermarket setup.
The bottom line is that an over-the-stack style filter will reduce power and torque in every application, and when well sealed, the side-mounted filters work well.
Leave it stock.
MODIFICATION OF THE AIR INLET SCREENS
Intake air must first travel through the cowl inlet openings and air tubes before it reaches the throttle body. The highest restriction of intake air flow area on a 748/9X6 appears to be at the upper cowl inlet openings, so a good first step is to remove the stamped steel decorative screens in the cowl air intakes and replace them with stainless steel or aluminum window screening for an significant improvement in flow area. Use foam double-sided tape to secure the screens. Flow area at the screen is then at least doubled. You can improve flow area further by using a Dremel cut-off wheel to remove about 1/2" of the outer (extreme left or right) perimeter of the pie slice shaped openings. This gives about a 20% additional increase in air flow area.
The main function of the inlet screens is to prevent large insects and small rodents from entering the air tubes and clogging the air filter. The problem is that they themselves also quickly clog up with debris.
Here’s a easy way to keep them clean.
Cut an additional piece of window screening to fit the triangular intake opening. Leave an additional 1/4-in all around to make tabs that fold back and hold the screen away from the screen you previously secured to the back side of the fairing with double-sided tape. The air pressure at speed will keep them in place. Now all you have to do is to pull out the forward screens for cleaning. The rear screens remain clear of debris. Think of these screens as pre-filters.
AIR INTAKE RESTRICTOR BLOCKS
The low and mid-range throttle response of an engine is mainly dependent on intake air velocity that is controlled by the throttle body diameter and velocity stacks. Removing flow restrictions in the intake tubes will only affect fuel mixture at red-line level engine speeds and ONLY if the restriction is enough to limit air volume reaching the velocity stack... a plugged air filter for example can do this.
The rubber restrictor annulus on the superbike inlet runners are designed to reduce air induction noise to meet EPA limits. These are not inlet restrictors designed to limit performance like those used recently on some Japanese bikes to meet maximum speed limits. The superbike blocks merely increase the velocity of the air as it passes through the device but reflect sound waves back to the engine inlet. Said another way, the pressure drop across them is very low as opposed to a high pressure drop created by a dirty air filter.
Ducati engines are the result of countless hours of development work on a dyno. They are designed as a unit with air flow and exhaust flow optimized together for each engine configuration. They can easily meet EPA noise limits without any loss of performance.
When you change things like intake and exhaust configurations it’s a hit-and-miss proposition. Many times you make improvements in one RPM range and performance at other speeds decreases. Without dyno before-and-after checks, multiple changes can produce confusing results.
Also, seat-of-the-pants tuning can be misleading. If a change, say, reduces mid-range performance at the expense of high end power the engine feels more peaky and this feels like an improvement. But is it? Strange dips in dyno horsepower and torque curves can and do occur at speeds where you spend most of your time riding.
Also, keep in mind that modifying the air ducts and airbox will modify the ram air effect at speed. Pressurizing the airbox with ram air gives an appreciable increase in performance. Since dyno comparisons are taken at zero mph air speed, any comparison results can be misleading.
Consider too, that the computerized engine management system uses a fixed fuel injection metering scheme controlled by the EPROM chip that was developed in combination with the stock intake/exhaust configuration. Unless you install a programable FIM chip and sort out any changes on a dyno with a knowledgable operator/programmer you won’t get optimum (low-end?, midrange?, high-end?, power?, throttle response?) performance.
It might even get worse ...
Until you have an opportunity to do some before-and-after tuning on a dyno I'd leave them in. If you take out the restrictor blocks you’ll get a nice intake growl ... but (hopefully) nothing more.
RACE BIKE VS. STREET BIKE AIR FILTERS
An air filter is not motorcycle performance equipment, it is protective equipment. It's function is to prevent dirt from entering the engine and damaging it. Dirt, by nature, is very abrasive and gets caught between parts that require a precision fit to function correctly.
An air filter that is selected for use on a race bike most often is not a good choice for use on a street bike. A race bike's function is to provide maximum performance and to finish (win) the race. Often the life of a factory racing team engine is practicing for, and finishing, one race. Then it's rebuilt to restore clearances by replacing any worn parts.
So, an air filter that is used on a race bike is selected using different priorities than one that is selected for use on a street bike. It primarily has to minimize any adverse effect on engine performance while still preventing the engine from ingesting dirt from a controlled racetrack environment. It has to capture and hold enough dirt without reducing intake air flow (clogging) to finish the race. It has to be accessible enough to be changed or cleaned quickly under racetrack conditions.
A street bike air filter, on the other hand, needs to function for thousands of miles in a variety of dusty conditions before cleaning or replacement. Consequently, it needs to hold a lot more dirt, and doesn't need to be nearly as accessible. Performance degradation is important but is still secondary to filter life.
The smaller the dirt particles captured by the filter, the better. Some designs and materials are better at this (filter efficiency) than others.
For example, paper is a good for capturing very small dirt particles but once the particle gets caught, it plugs the hole (smaller than the dirt particle) in the paper, so the next particle gets stuck in an adjacent hole, and so on. Consequently, once all the holes are filled, no more air can pass and the filter's clogged up. Paper filters need to have a large enough area to have a reasonable life, so for this reason the paper is pleated to increase it's area and it's dirt-holding capacity.
Foam filters work differently. You have to oil a foam filter to trap small dirt particles smaller than the foam air spaces. Otherwise they pass right through.
Foam air filters are a reasonable balance between good airflow, dust holding capacity, and filtration efficiency for small particles. The way foam air filters work is simple. Open cell polyurethane foam is wetted with a specially developed sticky oil. The sticky oil is suspended in the path of the dirty air on the strands of the web-like cell structure of the foam. This makes it difficult for small dirt particles to pass through the depth of the filter without sticking to the strands. Larger particles are trapped if they are bigger than the distance between the cell strands themselves.
As the outer wetted strands become loaded with dirt particles and no longer sticky, the wetted strands downstream continue trapping dirt, until the entire foam thickness is utilized. Also as dirt particles build up on the strands the space between strands decreases, further increasing the filter efficiency by trapping the dirt particles that initially could pass between the strands in a clean filter. This approach prevents the surface loading and air restriction that single-stage paper filter elements experience and consequently extends the service life of the air filter element. Finally, when the filter is sufficiently dirty to stop trapping small particles and clogging reduces airflow, it can conveniently be washed, re-oiled, and re-used.
You can even combine such materials into a multi-stage filter. Stage One could be window screening to keep out large debris down to insect-size material, Stage Two could be open-cell foam, and Stage Three (just before the engine inlet) could be paper. This approach prevents the paper from being clogged with dirt that could be stopped by either the foam or the screen.
Whatever approach you or the filter manufacturer take, the underlying issue is that you need to CLEAN the filter before accumulated dirt reduces airflow sufficiently to reduce engine performance and economy.
As some of you know, I've long been recommending that you keep your stock filters mainly because they have fewer drawbacks than the aftermarket filters, and they offer a performance advantage. However, they do have one significant drawback themselves and that is they're made using foam that has a uniform pore size that is run without using a dirt retention oil. So, they're not a very efficient filter, but they flow well.
I've also been critical of the marketing hype by aftermarket filter manufacturers and their failure to provide dyno results that demonstrate that their product provides a performance advantage.
Well, all that's been changed ...
I've just finished reviewing the Pipercross dyno chart for the MPX038 superbike filter and the web information about the filter's construction and I believe that this filter is currently the BEST FILTER available when used with their dirt retention spray and cleaned regularly.
Further, I would appreciate any help anyone can provide as to where to buy for shipment to the US. The manufacturer's price including VAT for the pair is £23.50/$34.00 but Foxy indicated he found them for £16.00. So any help you can give us Yanks will be appreciated.
Yep, I purchased them from Demon Tweeks.
The exact price was £16.50 and £11.69 for the cleaning kit.